#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<math.h>

#define N 100
int i,j;

int main(void)
{
	int n;
	double pai,tmp1=0.0,tmp2=0.0;
	for(n=1;n<=N;n=n+2)
	{
		tmp1 = tmp1 + 1.0/(2*n-1);
	}
	for(n=2;n<=N;n=n+2)
	{
		tmp2 = tmp2 - 1.0/(2*n-1);
	}
	
	pai = 4.0*(tmp1+tmp2);
	
	
	printf("%.8lf",pai);
	
	return 0;
}
